1. Field of the Invention
This invention relates to a measuring method and a measuring device which can be advantageously applied to a positioning device for positioning a mask relative to a wafer in an exposure apparatus for producing semiconductors or the like and to a superimposition accuracy measuring device.
2. Description of the Related Art
FIGS. 1 and 2 show a conventional positioning device, which is disclosed in Japanese Patent Application Laid-Open No. 64-8264. The device shown uses the optical heterodyne method, in which linear diffraction gratings are used as positioning marks. FIG. 3 illustrates the principle of this device. Referring to FIG. 1, a ray of light 202 emitted from a dual-channel linear-polarized laser beam source (Zeeman laser) 201 impinges upon a diffraction grating 203, and diffracted rays of light from this grating, i.e., a zero order diffracted ray of light 206, a +1st order diffracted ray of light 205 and a -1st order diffracted ray of light 207, are deflected by a mirror 204. After passing through an illumination optical system 208, one of the three diffracted rays of light is cut off. Then, the direction of polarization of one of the two remaining diffracted rays of light is changed by a half-wave plate, and applied, at an application angle as determined by the numerical aperture of the illumination system 208, to a diffraction grating 211 on a mask 212 and to a first diffraction grating 218 above a wafer 213. The travelling direction of a first diffracted ray of light 215, generated by reflection and diffraction at the first diffraction grating 218, is the same as the travelling direction of a second diffracted ray of light 216, generated by reflection and diffraction at the diffraction grating 211. However, as shown in FIG. 2, the position of the first diffraction grating 218 is offset in the y-direction with respect to the position of the diffraction grating 211, so that the first and second diffracted rays of light 215 and 216 do not coincide but instead are slightly separated from each other.
Numeral 220 indicates a first detector which is adapted to receive exclusively those components of the first diffracted ray of light 215 from the first diffraction grating 218 which have the same direction of polarization and are separated by a polarization beam splitter 223. The second diffracted ray of light 216 from the second diffraction grating 211 is intercepted by a knife edge 221 and does not impinge on the first detector 220. Numeral 224 indicates a second detector adapted to receive exclusively those components of the second diffracted ray of light 216 from the second diffraction grating 211 which have the same direction of polarization and are separated by a polarization beam splitter 223. The first diffracted ray of light 215 from the first diffraction grating 218 is intercepted by a knife edge 222 and does not impinge on the second detector 224. Numeral 225 indicates a phase difference meter for detecting the phases of optical beat signals which can be detected by the first and second detectors 220 and 224; numeral 226 denotes a wafer stage driving circuit for a wafer stage 214; and numeral 227 denotes a mask stage driving circuit for a mask stage 219.
Here, the principle of the positioning of the mask 212 relative to the wafer 213 will be illustrated with reference to FIGS. 3(a) and 3(b).
Referring to FIGS. 3(a) and 3(b), a synthesized light E.sub.M of the -1st order diffracted ray of a ray of light having a frequency of f.sub.1 and the +1st order diffracted ray of a ray of light having a frequency of f.sub.2 can be expressed by the following equation: EQU E.sub.M =A.sub.1 exp{i(2.pi.f.sub.1 t-.delta..sub.M)}+A.sub.2 exp{i(2.pi.f.sub.2 t+.delta..sub.M)} (1)
where .delta..sub.M =2.pi..multidot..DELTA.X.sub.M /P (P: the pitch of the grating) and A.sub.1 and A.sub.2 are constants determined experimentally. Symbol .delta..sub.M represents a variable generated by a displacement of the first grating 218 by an amount of .DELTA.X.sub.M in the x-direction with respect to a reference position.
The intensity I.sub.M of the synthesized light E.sub.M expressed by equation (1) can be expressed by the following equation: EQU I.sub.M =.vertline.E.sub.M .vertline..sup.2 =A.sup.2.sub.1 +A.sup.2.sub.2 +2A.sub.1 A.sub.2 cos {2.pi.(f.sub.1 -f.sub.2)t-2.delta..sub.M }(2)
The phase of the optical beat signal given by equation (2) varies by an amount corresponding to a change in the component which is expressed by the third term of equation (2) and which is generated during the displacement by .DELTA.X.sub.M of the first diffraction grating 218, that is, by 4.pi..DELTA.X.sub.M /P (radian).
Since the value of P is known, it is possible to detect the displacement .DELTA.X.sub.M of the first diffraction grating 218 by detecting changes in the phase of the optical beat signal.
In completely the same manner as described above, it is also possible to detect the displacement .DELTA.X.sub.W of the second diffraction grating 211 on the wafer 213.
The optical beat signal I.sub.W detected by the second detector 224 represents the intensity of the synthesized light of the 1st order diffracted light having a frequency of f.sub.1 and the -1st order diffracted light having a frequency of f.sub.2, and can be expressed by the following equation: EQU I.sub.W =A.sup.2.sub.1 +A.sup.2.sub.2 +2A.sub.1 A.sub.2 cos {2.pi.(f.sub.1 -f.sub.2)t-2.delta..sub.W } (3)
where .delta..sub.W =2.pi..multidot..DELTA.X.sub.W /P.
The phase difference .DELTA..phi. between the optical beat signal which is represented by equation (2) and which can be detected by the first detection means 220 and the optical beat signal which is represented by equation (3) and which can be detected by the second detection means 224, can be expressed by the following equation: EQU .DELTA..phi.=4.pi.(.DELTA.X.sub.M -.DELTA.X.sub.W)/P (4)
In this way, the phase difference between the mask-diffracted-ray optical beat signal and the wafer-diffracted-ray optical beat signal is detected, and the mask stage and the wafer stage are moved relative to each other in such a way that the phase difference becomes 0.degree., thereby accurately positioning the mask relative to the wafer.
In actually measuring and evaluating the positioning performance of a device constructed as an exposure apparatus, a minute pattern formed on a mask has conventionally been superimposed on a wafer and transferred thereto by printing, and the displacement of the pattern on the wafer has been measured. For example, a vernier measurement method has been well known in the art, in which, as shown in FIG. 4(c), a so-called vernier pattern is formed on a wafer through exposure, and any displacement thereof is observed by magnifying it with a microscope or the like. The patterns in FIGS. 4(a) to 4(c) are all resist patterns exposed on wafers, the resist existing in the hatched sections. FIGS. 4(a)-4(c) show a means measuring displacement in the x-direction only. Here, patterns 251 and 252 form verniers with respect to each other, one division of which corresponds to 0.05 .mu.m. First, exposure is effected through a first mask (reticle) onto a wafer to form (develop) the pattern 251, and then resist is applied thereto. Further, a second mask (reticle) having the pattern 252 is aligned with respect to the wafer and the exposure is effected, thereby forming the pattern 252 on the wafer. Then, it is checked to what degree of accuracy the first and second masks (reticles) are superimposed one upon the other after the alignment. The check is effected through measurement of the vernier pattern on the wafer, in which the patterns 251 and 252 are printed together as shown in FIG. 4(c), by magnifying the vernier pattern on the wafer by a microscope. For example, in the case of FIG. 4(a), the pattern 251 has a pitch of 7.95 .mu.m and in FIG. 4(b) the pattern 252 has a pitch of 8.00 .mu.m. The measurement and evaluation of superimposition for printing in conventional semiconductor exposure apparatuses have been conducted in the manner described above.
However, with the above second prior-art example, the measurement operation is difficult to automate and requires many hands. The first prior-art example allows automation of the measurement to be easily effected. On the other hand, it involves the following problems.
1 Due to scattering and deterioration in the profile irregularity at the edge portions of the knife edges for splitting the two interference rays of light, noise is included in the beat signals.
2 Any displacement of the set positions of the knife edges for splitting the two interference rays of light affect the measurement accuracy.
3 Since half the rays of light are cut off, a large loss in the quantity of light is involved.